With no motive, suspects or arrests, the death of Nairn banker Alistair Wilson, gunned down on his own doorstep by a mystery assassin, is one of Scotland's most baffling murder cases.

It is 15 years today since the young father was killed.

He was interrupted while putting his two young sons to bed and shot at point-blank range.

A former London detective who has followed the case closely claims local police lacked knowledge about financial crime that he believes may have helped catch the killer.

Alistair Wilson, 30, was widely regarded as a talented banker with everything to live for - a good job, happy marriage and two sons he adored.

The Nairn family were torn apart on November 28, 2004.

While putting their boys to bed for the night, a stocky man knocked at their door, offered a mystery envelope and shot Mr Wilson three times. He died en route to hospital.

The rare 1920s handgun surfaced in a drain ten days later, a few streets away.

The motive is unknown. Theories abound that it was related to Mr Wilson's role with the Bank of Scotland.

Retired Metropolitan Police detective Peter Bleksley claims police lacked a basic understanding of the financial world.

He told STV News: "I would like to have seen specialist financial investigators, that the investigation went far, far deeper than it actually did.

"I fear that when the police went to the bank in the first instance and said 'tell us what you know' that the police got what they were given rather than perhaps probing more deeply, seizing computers, seizing servers even, and saying 'we're shutting this down until we get to the bottom of it'."

Nevertheless, almost 4,000 statements were taken and police say they remain committed to the investigation.

They acknowledged the family "still wants and deserves answers".

In a statement, Police Scotland said: "Those reluctant to come forward with information previously may be more willing to do so," adding that "someone, somewhere, knows why Alistair was killed and who was involved".

Tom Heggie, a former tutor of the Wilsons' elder son has urged anyone with information to come forward.

He said: "I have come to know members of the family, particularly through being a teacher at Nairn Academy. I think they require some closure on this because each year it's a kind of reopening of old wounds.

"And I think if anybody out there knows anything then, apart from anything else for the sake of the family, perhaps there should be some sort of resolution to this."

His plea echoed that of Mr Wilson's widow Veronica in December 2004.

Standing outside police headquarters in Inverness at the time, she told journalists: "I don't know why anybody has done this to us and, if nothing else, for the sake of my two young boys, we need to find out why.

"So, if anybody has any information please come forward."

The crime still resonates in Nairn and much further afield.

Former Nairn Provost Sandy Park said this week: "It doesn't matter where you go. I've even been on holidays abroad and people say 'you're from Nairn. Oh, that's where that murder was. Have they found the murderer yet?'

"So, it's still there, the local people still remember it. It would be great if we could draw a line and find somebody who did it."

Story by Iain Ramage